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Author Topic: Dual battery systems, AGM batteries and charging  (Read 3027 times)
AB2MH
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« on: June 13, 2004, 07:08:25 PM »

I'm seriously considering a dual battery system for mobile HF and to power a 1000 watt inverter.  AGM Golf cart deep cycle batteries are my choice, since they provide excellent deep cycle performance.

But since they're not like wet cell batteries, their charging voltage and current is different.

Has anyone used AGM batteries and had them charged from a car's alternator?  If so, any pointers?  I wanted to use an isolator, since that would separate the auxiliary battery from the starting battery.  But I don't want to burn my alternator, and I'm trying to avoid getting a high output alternator (although I figured that eventually this will be inevitable).
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K0BG
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« Reply #1 on: June 14, 2004, 09:58:33 AM »

Depends on how much charging current you wish to have. Since the batteries are of different types, an isolator is probably a good idea. There are also special charging isolators available for doing exactly what you want to do, but they are not cheap! Most good RV dealers sell these devices.

I suspect you want to run your radio off of the inverter. This can be problematic depending on the radio and the inverter you use. In most cases, it is better to use the 12V directly. However, when the battery voltage drops below 11.5 or so, most rigs will just shut down.

As far as the alternator; most newer vehicles have 100 amp or larger units. If you're not using the headlights, there's 30 amps or so for you to play with. Fact is, this is the last item you should be worring about.

Alan, KØBG
www.k0bg.com
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K5LXP
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« Reply #2 on: June 14, 2004, 10:35:29 AM »

AGM's could be charged by an alternator but it probably wouldn't be the best way.  Vehicular charging systems only go to 14.2V or so, but to properly finish charge an AGM you need to float them at 15-15.5V.  You need to consult the data sheet for the particular AGM's you're interested in, but generally speaking they need a pretty specific and accurately controlled 3-stage charge algorithm, which automotive charging will not provide.  Especially during the finishing portion of the charge, where voltage and current must be just right or you get an incomplete charge, or cause the battery to vent.  Flooded cells are more forgiving with overcharging, AGM's are not.  If you can accept a diminished capacity and lifespan for the AGM you could probably just connect it up as a second isolated battery.  Better would be to connect it to a proper charger while the vehicle is idle.  Depending on how many Ah's we're talking about (and with a 1kW inverter, you're going to need a few) you would need to run your alternator for many hours to completely recharge your AGM.  Normal driving cycles likely wouldn't provide enough recharge time depending on how deeply you discharge the AGM.

In short, I would either consider a different chemistry compatible with a vehicular system or incorporate a proper charger in the vehicle that will accomodate the AGM.

Mark K5LXP
Albuquerque, NM

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AB2MH
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« Reply #3 on: June 14, 2004, 02:27:55 PM »

Thanks for your replies.

Regarding Alan's question, no, of course I'm not going to run my ham rig off an inverter.  The inverter is for occasional, non-ham use when I'm out far from home.  My HF gear is going to be powered by strictly 12 volts DC.

The AGM battery I'm considering is a 110Ah battery (20 hour rate).
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KC2MMI
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« Reply #4 on: June 18, 2004, 09:42:24 PM »

Ryan, I'm doing this now with #4AWG cable to the AGM in the trunk.

"Proper" AGM charging voltages & protocols vary with each manufacturer. Most AGMs today are sufficiently close to automotive batteries to use in most cars. I say "most" because some cars still charge at 13.8-14.4 while others charge at very precisely 14.3-14.4.

So, you need to get the specs for your AGMs and the exact spec for your car.

The Optima Redtops, for example, are AGMs designed for direct use in cars, same system as wet cells, no problems.

If anything an AGM typically wants just a little less voltage (0.1-0.2V) less than a car battery and the 40' of cable tying them together will supply that.<G> I use dual cables, running my ground to the primary battery ground, because I don't want to know about ground loops.

I'd suggest looking at a West Marine battery combiner, too. Overpriced little box that keeps both of your batteries isolated until the primary battery is seeing 13.4(?) volts, and then it parallels the two so both can charge. With manual override and bypass. Like everything else, available on eBay. I know, it seems overpriced, but I couldn't find a good source for a high-power relay, and these things have a sterling reputation.

(If you want to give me a call, my number is in the group web site database.)

Been working a lot with high power DC, from boat systems. This is essentially the same, except hams don't always have problems from salt air corrosion.<G>

If you need cable...I've still got over 50' of fully tinned #4AWG Type 3 machine wire, very similar to battery cable, that I was going to post on eBay next week. Needed it for the car, and a total rewire job on a friend's boat. (Which will never have an electrical problem again.<G>)

The hardest part of the whole job is swaging terminal lugs on #4 cable!
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AB2MH
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« Reply #5 on: June 20, 2004, 07:09:33 PM »

Thanks Jared,

I may take you up on your offer, but I'll check out the west marine battery combiner.  

As for getting the terminal lugs on the ends, just use screw terminals.  These are available from home depot and clamp on securely to the ends of the wire.  You need an allen wrench to tighten it.
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AB2MH
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« Reply #6 on: June 29, 2004, 09:26:15 PM »

Well I did some voltage monitoring and basically the voltage tops out at 14.25 volts and remains pretty steady.

I believe the car has a 100 amp alternator as standard equipment.

So far charging the AGM battery has shown no problems to either my car's electrical system or the battery itself.
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